Unlike the primary battery which is not chargeable, the secondary battery is a battery that can be repeatedly charged and discharged. The secondary battery is used as a power source not only for compact and portable electronic devices such as mobile phones, PDAs, laptop computers, and so on, but also for electric vehicles or hybrid vehicles.
The secondary battery is fabricated and used in a form of a cell sealingly enclosing therein battery components such as positive electrode, negative electrode, separator, electrolyte, and a variety of additives. Meanwhile, the secondary battery may have increasing thickness of the battery cell as the chemical reaction from the repeated charging and discharging causes changing volumes of many materials including electrode active material, electrolyte, additives, and so on, generation of gas, and the like. Further, such change in the thickness of the battery cell can occur due to the heat generated during charging and discharging, the heat generated from abuse of the battery, such as over-charging, over-discharging, and so on, and also due to the heat when the battery is subject to high temperature environment.
Meanwhile, the battery cell is categorized into a can-type battery and a pouch-type battery according to the type of the case used. The can-type battery encloses the components mentioned above in a metallic rectangular or cylindrical case, and the pouch-type battery encloses the components mentioned above in a pouch consisting of a sheet with aluminum as a main material and synthetic resin coat layer laminated thereon. Although the can-type battery is physically stronger than the pouch-type battery, the pouch-type batteries are recently more widely used for relatively light weights and ease of manufacture thereof. However, due to lower physical strength, the pouch-type battery has a shortcoming that it is susceptible to the thickness change of the battery cell mentioned above.
As the thickness of the battery cell increases, that is, as the internal pressure of the battery cell increases, the battery cell can break starting from the weakest portion of the case, which may lead into accident such as fire or explosion of the battery cell. Accordingly, the pouch-type battery cell is tested by performing a predetermined number of charging and discharging cycles or leaving the cell for a predetermined time under high temperature environment, and then measuring a thickness of the battery cell to thus determine the suitability of the materials such as electrolyte, additives, and so on, and also the sealability of the pouch-type battery cell. Further, Korean Patent No. 10-1397926 discloses placing a flexible pouch-type battery cell into a vacuum chamber and measuring a thickness of the battery cell before and after the vacuum state is formed with a vacuum pump, and estimating the sites likely to have electrolyte leaks, i.e., testing sealability of the pouch-type battery cell using the difference in the thickness.
Meanwhile, the pouch-type battery cell is not used directly as in the flexible pouch form. That is, the pouch-type battery cell is either received in a hard external case to be used as a power supply battery for a portable electronic apparatus, or fabricated into a module-type battery in which a plurality of pouch-type battery cells are stacked in a hard frame for use as power storage system or as a high-capacity battery such as a power source of an electric vehicle or a hybrid vehicle. Accordingly, the pouch-type battery cell, either when it is in use or in storage, is subject to a constant pressure due to the external case or the frame. Meanwhile, in the related art, the thickness of the flexible pouch-type battery cell is measured simply in a fixed condition only. Accordingly, the related art has a limit that it cannot observe and predict changes in the thickness of the battery cell as being in actual use or in storage.